1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an alignment layer for initial alignment of liquid crystals in an LCD device.
2. Discussion of the Related Art
Among ultra thin flat panel display devices having a display screen of a thickness of several centimeters, LCD devices have been widely used for notebook computers, monitors, spaceships, aircrafts, and other applications due to their features and advantages of low driving voltage, low power consumption, portability, and the like. Generally, an LCD device includes a color filter substrate having color filter layers formed thereon, a thin film transistor substrate facing the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer formed between these substrates.
In such an LCD device, alignment of the liquid crystal layer is changed by applying a voltage to control transmittance of light, thereby generating an image. In order to affect the alignment of the liquid crystals, electrodes are formed on the thin film transistor substrate and/or the color filter substrate for application of voltage. In a twisted nematic (TN) mode LCD device, for example, a pixel electrode is located on the thin film transistor substrate and a common electrode is located on the color filter substrate so as to generate a vertical electric field between the two substrates. In an in-plane switching (IPS) mode LCD device, for example, the pixel electrode and the common electrode are located parallel to each other on the thin film transistor substrate so as to generate a horizontal electric field.
FIG. 1 is an exploded perspective view illustrating a related art TN mode LCD device. As shown in FIG. 1, a thin film transistor substrate 10 includes a gate line 12, a data line 14 crossing the gate line 12, a thin film transistor T formed at a crossing region of the gate line 12 and the data line 14, and a pixel electrode 16 connected to the thin film transistor T. A color filter substrate 20 includes a light-shielding layer (or black matrix) 22 to prevent leakage of light, R, G, and B color filter layers 24, and a common electrode 25 formed on the color filter substrate 20. In this manner, a vertical electric field is generated between the pixel electrode 16 on the thin film transistor substrate 10 and the common electrode 25 on the color filter substrate 20, thereby allowing alignment of liquid crystals to be controlled.
The substrates 10 and 20 constructed as described above are bonded to each other to form a single liquid crystal panel with a liquid crystal layer formed between the substrates 10 and 20. If the liquid crystal layer is randomly aligned between the substrates 10 and 20, it is difficult to achieve a consistent arrangement of molecules in the liquid crystal layer. Thus, although not shown in the drawings, an alignment layer for the initial alignment of liquid crystals is formed in the thin film transistor substrate 10 and/or the color filter substrate 20.
Examples of a method for forming an alignment layer for initial alignment of the liquid crystal include a rubbing alignment method and a photo-alignment method. In the rubbing alignment method, after an organic polymer such as polyimide is thinly coated on a substrate, a rubbing roll wound with a rubbing cloth is rotated on the coated substrate to rub the organic polymer, thereby arranging the organic polymer in a uniform direction.
However, the rubbing alignment method has the following drawbacks. Contaminant materials, such as dust, may be attached to the surface of the substrate due to static electricity generated by friction between the rubbing cloth and the organic polymer. Further, when the arrangement of the rubbing cloth becomes misaligned, a problem of light leakage may occur. Such problems with the rubbing alignment method are caused by the mechanism for providing physical contact between the rubbing roll and the substrate.
In order to solve the problems of the rubbing alignment method, various studies have been conducted for providing a method for manufacturing an alignment layer without the need of physical contact. Thus, instead of using the rubbing alignment method, use of a photo-alignment method has been suggested. The photo-alignment method produces an alignment layer by irradiating polarized ultraviolet (UV) rays onto an alignment material having a photo-reaction group. In order to align the liquid crystals, the alignment layer must have an anisotropic structure, which can be formed when the photo-reaction group is anisotropically reacted with the polarized UV rays.
Although the photo-alignment method may address the problems related to the rubbing alignment method described above, the photo-alignment method has a serious problem in that anchoring energy is low, thereby creating an afterimage. More specifically, because polarized UV rays are used in the photo-alignment method, only the photo-reaction group identical with the polarized direction of the UV rays undergoes a photo-reaction, thereby failing to form a desired anisotropic structure.
In particular, the alignment material coated on the substrate is in a random arrangement having no particular direction. Therefore, a response rate is reduced and too little anisotropy obtained if only the photo-reaction group identical with the polarized direction of the UV rays undergoes the photo-reaction. As a result, the photo-alignment method provides lower anchoring energy and causes a problem of afterimage effects. In order to solve the problems caused by the reduced response rate in the photo-alignment method, various methods have been studied.
First, there is a method for improving the response rate by heating a substrate when irradiating UV rays. However, this method is not suitable for mass production because arranging a heating device to heat the substrate becomes difficult if the size of the substrate becomes large.
Second, there is a method for improving a response rate by increasing density of a photo-reaction group of an alignment layer. However, this method causes too high reaction group. In such as case, liquid crystals are not aligned well due to steric hindrance caused by such high reaction group.
Finally, there is a method for improving a response rate by using an alignment layer having a low molecular weight. However, this method fails to provide stable alignment of liquid crystals.